1. Field of the Invention
The present invention relates to production of an optical disk master and an optical disk, and more particularly to a master disk production method suitable for production of a high density data recording optical disk and this type of optical disk.
2. Description of the Related Art
An optical disk as been put to use as a high density data recording medium. Formation of a relief shaped pattern (diffraction structure) comprised of pre-pits and guide tracks formed in the optical disk begins with production of a master disk in accordance with the procedure as shown in FIGS. 10A to 10C. Conventionally, a positive type photoresist layer 5 is formed on a glass substrate 6 by spin coating as shown in FIG. 10A and a light beam modulated in intensity by a signal to be recorded (a signal for formation of pre-pits such as addresses and clock or a signal for formation of guide tracks) is focused by means of a recording lens 17 of a master disk cutting machine to expose the photoresist layer 5. At that time, latent images 32' and 33' are formed in the photoresist layer 5 corresponding to an intensity distribution 31' of the focused optical spot. By setting the threshold of development to a value which prevents the latent image 33' due to a higher order diffracted beam from being developed, a guide groove 8, for example, can be formed. Actually, since the higher order diffracted beam is very weak and its latent image 33' is also weak, the conventional development process in which surface roughness of the photoresist layer is suppressed will do. A stamper is produced from this master disk by plating and many replica disks are made using the stamper. A recording film is deposited on the replica disk and the replica disk is finished to complete an optical disk. A method of making this type of optical disk is described in, for example, an article "Manufacture of Laser Vision Video discs by a Photopolymerization Process" in Philips Technical Review, Vol. 40, No. 10, 1982, pp 287-297.
The relief shape (diffraction structure) of the pre-pit and guide track of the master disk is determined by the optical spot intensity distribution of the cutting machine and the development condition. Narrowing of the optical spot diameter of the zero-th diffracted beam is limited by a diffraction limit of the recording lens and therefore formation of a master disk having a relief pattern (diffraction structure) of a narrower width than the diffraction limit is difficult to achieve. At present, an Ar laser of 458 nm beam or a He-Cd laser of 442 nm beam is used as light source and a positive type resist is used as photoresist. The recording lens of the cutting machine has a numerical aperture NA of about 0.9 and hence, the optical spot diameter measures about 0.5 .mu.m which approximately equals the lower limit of the width of the pit and track.